Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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Eichornia crassipes (water hyacinth) is an introduced species that grows very rapidly and

has become a nuisance in the southeastern United States.

Phragmites and cattails are among the most productive wetlands plants, having photo-

synthetic efﬁciency from 4 to 7%. Measurements of primary production ranges from 1000

to 6000 g/m

yr for Phragmites and 2,040 to 3,450 g/m

yr for Typha. However, the pat-

tern differs for these two plants. Typha grows fastest early in the growing season, then

slows down. Phragmites grows at a fairly constant rate throughout the season.

The most obvious invertebrates in the freshwater marsh are members of the insect

order Diptera, the true ﬂies. These include the mosquitoes as well as midges and crane

ﬂies. Many are herbivores, although many others are benthic in their larval stages.

Most of the mammals are also herbivorous. One of the most common mammals is the

muskrat (Ondatra zibethicus).

Omnivorous and herbivorous waterfowl are plentiful. They nest in northern marshes

and migrate between them and southern marshes in the winter. The loon frequents deeper

ponds where there may be ﬁsh. Some ducks nest in upland locations; those that ﬁsh by

diving nest over the water. Geese, swans, and canvasback ducks are the major herbivores

in the marsh. Wading birds nest in colonies in marshes and ﬁsh in shal low waters. Song-

birds and swallows usually nest and perch in uplands near the marsh, and ﬂy into the

marsh to feed.

Marshes may provide shelter for reproduction of ﬁsh from adjacent lakes. Common

carp (Cy prinus carpio) are able to withstand great ﬂuctuations in water temperature

and dissolved oxygen that occurs in shallow marshes. They graze and uproot the vegeta-

tion, increasing turbidity in the water.

Freshwater marshes tend to be both nitrogen and phosphorus limited. Vegetation plays

a role in nutrient cycling similar to that in forests. They remove nutrients from the soil,

only to return them as dead plant mater ial is degraded.

Riparian Wet lands The riparian zone is the area adjacent to rivers that has a high water

table and is regularly ﬂooded by the river. Also called the ﬂoodplain, they are widely dis-

tributed, even in arid areas. They are open systems, with signiﬁcant exchanges of energy

and material with the adjacent upland and riverine ecosystems, as well as upstrea m and

Figure 15.22 Zonation of vegetation in a freshwater marsh. (From Mitsch and Gosselink, 1993.

Used with permission.)

548

ECOSYSTEMS AND APPLICATI ONS

downstream areas. Because of their wide distribution and narrow geometry, they have

been particularly vulnerable to draining and ﬁlling. Riparian wetlands provide water

storage during ﬂood stages of the river. The elimination of this ‘‘safety valve’’ has

contributed to downstream damage due to ﬂoods.

Western riparian wetlands of the United States tend to be narrow and have steep geo-

morphology. The uplands tend to be clearly distinguished from the wetlands. Eastern

riparian systems include those of the Mississippi valley, the southeastern and northeastern

United States. They may have steep morphology along low-order streams near the high-

elevation headwater. The higher-order streams at lower elevations tend to have gentler

slopes, wider wetlands, and more gradual transition to uplands. A natural levee often

forms by sediment deposition between the wetland and the river.

As ﬂooding is intermittent, anoxic conditions in the soil occur only when inundated.

The organic matter content is intermediate, about 2 to 5%. Clay deposits and import by

ﬂooding results in accumulation of a large quantity of nutrients.

Spatial and temporal scales are linked, as distance from the river (or actually, eleva-

tion) determines the frequency of ﬂooding. Figure 15.23 shows an example of this in

terms of a classiﬁcation applied to southwestern U.S. bottomland forest wetl ands. The

type of vegetation correlates approximately with the zone. For example, among the

oaks, overcup oak (Quercus lyrata) is typical of zone III, laurel oak (Quercus laurifolia )

of zone IV, and water oak (Quercus nigra) in zone V.

In arid areas, riparian wetlands are easy to distinguish from upland ecosystems, since

the latter are usually grasslands or deserts and the former are forested. They tend to form

narrow strips along the river, snaking their way through the otherwise dry region.

Obviously, they serve as a local oasis to animals and humans. These wetlands usually

lack oak but include willow (Salix spp.) and cottonwood (Populus fremontii), which are

also common throughout the United States. Western riparian wetlands also have sycamore

(Platanus wrightii), ash (Fraxinus pennsylvanica velutina), and walnut (Juglans major),

as well as alder (Alnus tenuifolia) at higher elevations.

Riparian wetlands often provide the only woodlands in an area, either because the

uplands have been converted to agriculture or because it was that way in the ﬁrst

place, as in arid areas. This increases the importance of riparian wetlands for providing

food and habitat, and as corridors for dispersal or migration.

Animal life in riparian wetlands is too diverse to describe here . Most are common to

adjacent upland or aquatic systems as well. Some are strongly associated with wetlands,

such as the beaver and the cottonmouth snake. In the western United States there are

88 species of birds that are strictly riparian. In southern California there are 45 riparian

mammals. Some species of ﬁsh spawn in the ﬂoodplains during ﬂood stage.

The intermittent nature of riparian ﬂooding augments the ecosystem productivity by

providing moisture without causing long periods of anoxic soil conditions, while provid-

ing nutrients and ﬂushing wastes away. Typically, riparian ecosystems with regular wet

and dry periods have an aboveground net biomass productivity exceeding 1000 g/m

 yr.

Riparian ecosystems are open systems. Much of their productivity is exported to the

adjacent waterway, and they can be sinks for nutrients from upla nd ecosystems.

15.3.7 Wetland Law and Management

In the United States, federal regulation of wetlands is not governed by any single piece of

legislation, as is the case with air and water pollution. Instead, wetlands issues are covered

WETLANDS 549

Figure 15.23 Zones of a southeastern U.S. riparian wetland. (Based on Mitsch and Gosselink, 1993.)

550

by a variety of laws related to water pollution and ﬂood control, plus Executive Orders.

The major relevant law is Section 404 of the Federal Water Pollution Control Act of 1972

(the ‘‘Clean Water Act,’’ PL 92-500). This section, without mentioning wetlands, gave the

Army Corps of Engineers authority to regulate dredging and ﬁlling in US waters. Cou rt

decisions plus another Executive Order expanded this auth ority to wetlands.

This led to the problem of creating a legal deﬁnition of a wetland. Wetlands identiﬁ-

cation and delineation is the process of determining whether a property is a wetland and

identifying the boundaries of the wetland. The primary guide for this purpose is the Corps

of Engineers Wetlands Delineation Manual (U.S. Army Corp of Engineers, 1987). Iden-

tiﬁcation and delineation are best done during the growing season. A site is classiﬁed as a

wetland only if it possesses indicators of wetland hydrology, hydric soil, and hydrophytic

vegetation. Hydrologic indicators include the actual presence of water at or near the soil

surface, and stains from ﬂooding on the trunks of trees. Hydric soil indicators include the

presence of mottling below the surface.

For the purpose of wetlands identiﬁcation and delineation, an area possesses hydrophy-

tic vegetation only if the dominant vegetation have one or more of the special adaptations

for survival in saturated soil conditions, such as were described in Section 15.3.2 . The

Manual places plants into several categories, based on the estimated probability that

they occur in wetlands under natural conditions (Table 15.21). Species that have an indi-

cator status of OBL, FACW, or FAC are considered to possess hydrophytic adaptations.

Exactly what is and isn’t a wetland must be prescribed very precisely for the purpose of

deciding whether legal restrictions apply. This is a matter of great importance to property

owners, because if they cannot develop their property, the property loses much of its eco-

nomic value. In 1992, the U.S. Supreme Court ruled that identifying a private parcel of

land as a wetland could constitute a ‘‘taking’’ of that land for the public good, and that the

landowner must therefore be compensated for the economic loss. Altho ugh this issue is

not resolved entirely, it could have a major impact on the ability of the government to

prevent further loss of wetlands.

The policy of ‘‘no net loss’’ has led to the strategy of creating wetlands to compensate

for the loss of wetlands due to pressing economic needs, such as highway construction.

Wetlands that are created where none existed before are called constructed wetlands.

Besides their use to compensate for losses, wetlands may also be construc ted to create

TABLE 15.21 Plant Indicator Status Categories

Wetlands Nonwetlands

Indicator Category Symbol Probability (%) Probability (%) Examples

Obligate wetland plants OBL > 99 <1 Spartina alterniﬂora,

Taxodium distichum

Facultative wetland plants FACW 67–99 1–33 Fraxinus pennsylvanica,

Cornus stolonifera

Facultative plants FAC 33–99 33–99 Gleditsia triacanthos,

Smilax rotundifolia

Facultative upland plants FACU 1–33 67–99 Quercus rubra,

Potentilla arguta

Obligate upland plants UPL <1 >99 Pinus echinata,

Bromus mollis

Source: U.S. Army Corps of Engineers (1987).

WETLANDS

551

habitats ﬁsh and wildlife, for ﬂood control, for nonpoint-source water pollution control, or

for wastewater treatment.

Wetlands, whether constructed or natural, purify wastewater by physically entrapping

particulate matter in the soil and organic litter, by the act ion of microorganisms on the

plants and in the soil, and by uptake directly by macrophytes, especially of nutrients.

Because the soil has both aerobic and anaerobic zones, wetlands can serve as a nitrogen

sink by both nitrifying and denitrifying. Wastewater treatment systems tend to use either

ﬂoating or emergent plants.

15.4 MARINE AND ESTUARINE ECOSYSTE MS

In contrast to the land , the oceans of the world are continuous with each other. Neverthe-

less, factors such as temperature, salinity, and lighting gradients isolate parts from each

other. The marine environments are those with salt or brackish water. Included in these

are the estuarine environments, which are bodies of water, partially enclosed by land,

in which fresh and salt water mix. Estuaries communicate with rivers and the ocean.

Saltwater marshes are often considered to be part of estuarine ecosystems.

A distinguishing feature of estuaries, besides having salinities intermediate between

fresh and ocean waters, is the existence of a salinity gradient in one of several forms.

In a vertically mixed estuary the salinity varies with distance from the mouth of the

river, but not with depth. A stratiﬁed estuary has salinity that varies both along the

axis of ﬂow and wi th depth, to varying degrees. In the most extreme case, salt water

ﬂows upstream along the bottom while brackish water ﬂows seaward along the surface.

This affects the transport of nutrients and of planktonic organisms. The salt wedge

estuary has salinity gradients with both depth and axially.

The oceans can be classiﬁed into two zones: the pelagic environment is occupied by

free-ﬂoating organisms; the benthic environment is the bottom subst rate. The benthic

environment includes the littoral zone, which is the part between the high-tide and

low-tide levels. Notice the difference from the use of this term for lakes, as deﬁned in

Section 15.2.1. Also, in oceans the term abyssal, which denotes the part of the ocean

deeper than the continental shelves, takes the role of the term profundal in lakes.

The pelagic zones of the oceanic province can also be distinguished based on

light penetration. Light penetration can be approximated as an exponential function

with depth:

I ¼ I

expða

zÞð15:9Þ

where I is the light intensity at wavelength l and at depth z, I

the intensity at the surface,

and a

the empirical extinction coefﬁcient corresponding to wavelength l. For example,

infrared wavelengths have a

¼ 20 m

1

in clear water. Using equation (15.9) to solve for

the depth at which the intensity is 1% of the surface yields z

¼ 0:23 m. Blue light is

weakly absorbed, with a

¼ 0:004 m

1

and a resulting z

¼ 1150 m. Red light is

absorbed intermediately, with a

¼ 0:4m

1

and a resu lting z

¼ 11:5 m. The total

light energy versus depth will thus be a complex sum of exponentials for the various

wavelengths (or, more accurately, an integral over wavelength). Overall, the extinction

of total light energy reaches 1% in water as deep as 110 m in clear tropical seawater,

or as shallow as only several meters in coastal waters.

552 ECOSYSTEMS AND APPLICATIONS

Overall, photosynthesis occurs in the top 100 m of the ocean, a portion of the epipe-

lagic zone called the photic zone. Below about 500 to 1000 m is the aphotic zone,in

which essentially no light exists. In between is the ‘‘twilight’’ or disphotic zone.

15.4.1 Productivity and Nutrients

Primary production in the open ocean is thought to be due mostly to phytoplankton.

Some 2 to 10% may be due to seaweeds, and less than 1% to chemoautotrophs. For all

the oceans as a whole, the gross productivity is estimated to be 50 to 100 g C/m

yr. Due

to surface scattering and light extinction, the efﬁciency of capturing incident sunlight is

only about 0.1 to 0.2%.

The phytoplankton are dominated by diatoms, and secondarily by dinoﬂagellates.

Although in the past most of the primary productivity of the oceans was attributed to

the larger phytoplankton, evidence is growing that as much as 70% of all the photosyn-

thetic activity in the oceans is due to smaller forms. These range from 2 to 20 mm in sizes

and are termed nanoplankton. They include the small single-celled plants called cocco-

lithophores. These are covered with plates of calcium carbonate. They can form deep beds

of ooze on the ocean ﬂoor, which fossilize into chalk formations such as the White Cliffs

of Dover in England. Another source of productivity is the phototrophic bacteria, such as

cyanobacter (not necessarily nitrogen-ﬁxing forms, however). Their abundance is of the

order of 100,000 mL

1

. They are from 0.2 to 2 mm in size and are termed picoplankton.

Productivity has an interesting relationship with depth in the epipelagic zone. Typi-

cally, the photosynthesis rate, and the concentration of chlorophyll, peaks some tens

of meters below the surface (Figure 15.24) . Several possible reasons are given for the

surprising observation that photosynthesis is not a maximum at the surf ace. One is that,

ironically, the light intensity may be inhibitory, causing bleaching of phytoplankton. It

may also be due to the effects of grazing by zooplankton. Mathematical modeling of phy-

toplankton growth dynamics suggests that nutrient limitations near the surface are respon-

sible. Nutrients continue to increase with depth up to on the order of 1000 m, and then

decrease modestly to the bottom. Oxygen reaches a minimum at similar depths, and then

increases steadily toward the bottom.

Depth (m)

Nitrate

Temperature

Primary production

Chlorophyll

Figure 15.24 Typical tropical structure of the water column. (Based on Mann and Lazier, 1991.)

MARINE AND ESTUARINE ECOSYSTEMS 553

If respiration is fairly uniform with depth, a point will occur where the rate of primary

production becomes equal to the respiration rate, yielding a net primary produc tion of

zero. This depth is called the compensation depth. It tends to occur approximately at

the depth where the light intensity has dropped to 1% of the surface intensity. This

depth deﬁnes the bottom of the euphotic zone. Below the compensation depth respiration

exceeds photosynthesis, and oxygen levels must be maintained by transport from surface

waters by mixing and advection. Figure 15.25 shows these relationships, and Figure 15.26

shows how they can be measured.

There are some interesting seasonal and geographical patterns in productivity as well

(Figure 15.27). One might expect that among open ocean waters, tropical oceans would

have the highest productivity. However, nutrients are always being lost to the euphotic

zone by the sedimentation of phytoplankton and zooplankton fecal pellets, and a perma-

nent thermocline prevents the nutrients from being mixed back upward. At 20



N latitude

the nitrate and phosphate concentrations are about 1% of the levels typical of temperate

oceans during the winter. In the tropics the nutrients lie mostly below 150 m depth, and

their concentrations peak at a depth of 500 to 1000 m. Productivity in most tropical waters

is less than 30 g C/m

per year. Exceptions to this situation are upwelling zones and reefs,

as noted below.

Polar zones, on the other hand, compensate for their low temperatures and low insola-

tion by a lack of stratiﬁcation. Thus, nutrients mix freely at the surface, except when

snowmelt forms a shallow layer of relatively fresh water. The shallow stratiﬁed layer

traps phytoplankton close to nutrients at the pycnocline (location of high vertical density

gradient), producing a very intense, though short-lived algal bloom. In the Antarctic

waters a unique circulation pattern produces upwelling of nutrients that stimulate produc-

tivity. However, even wi th these factors, the polar region productivity as a whole averages

less than 25 g C/m

 yr.

Depth (m)

100

Respiration rate

Compensation point

(marks bottom of

euphotic zone)

Net productivity

Maximum productivity

Productivity inhibited

by intense light at surface

Photosynthesis

rate

Figure 15.25 Productivity and respiration vs. depth, as would be measured using the light bottle/

dark bottle method. (Based on Garrison, 1993.)

554

ECOSYSTEMS AND APPLICATI ONS

In temperate and subpolar oceans, on the other hand, productivity is at a higher average

level, due to more dependable insolation and the nutrient replenishment that comes

with seasonal stratiﬁcation. Productivity peaks with a spring algal bloom, but it is mod-

erately high yearround. Typical productivity is around 120 g C/m

 yr, but can be as high

as 250 g C/m

 yr. Thus, these regions provide most of the productivity of the world’s

oceans.

In all climates, productivity as high as 1000 g C/m

 yr can be maintained where

hydrodynamic factors bring nutrients to the surface. Places where this occurs are called

upwelling zones. These are caused by wind-driven currents combined with the Coriolis

effect. Th e Coriolis effect is the tendency caused by Earth’s rotation for ﬂows to be

diverted to the right in the northern hemisphere and to the left in the southern. Thus, hemi-

sphere, the trade winds around the equator cause a westerly current in the Paciﬁc and

Atlantic oceans. The Coriolis effect produces a divergence of ﬂow, as the northern

edge diverts toward the north and the southern toward the south. The net effect is that

surface water ﬂows away from the equator, and deeper waters are brought up to replace

it. This phenomenon is called equatorial upwelling. The regions of high productivity

that result from the upwelling nutrients can be seen in Figure 15.27. A simi lar divergence

zone encircles the Antarctic, where ocean currents ﬂow in opposite directions, due to

Oxygen consumption

in light (transparent)

bottles includes the

sum of respiration

and photosynthesis.

Oxygen consumption

in dark (opaque)

bottles includes the

respiration only.

Photosynthesis

can be computed

as the difference

between the

two.

Figure 15.26 Light bottle/dark bottle method of measuring primary and net productivity vs. depth.

At the compensation point the dissolved oxygen (DO) concentration in the light bottle will not

change with time. Above that point, photosynthesis exceeds respiration and DO will increase. At

greater depth the DO will decrease with time. (Based on Garrison, 1993.)

MARINE AND ESTUARINE ECOSYSTEMS 555

trade winds. The resulting enhancement to productivity partially compensates for the

otherwise low productivity of the polar oceans.

Another important mechanism is coastal upwelling, in which winds parallel to the

coast, in combination with the Coriolis effect, produce an offshore current. This brings

the deeper waters to the surface with their nutrients. This occurs throughout the world

but is particularly massive in four locations: the coasts of Peru, California and Oregon,

northern Africa, and southwestern Africa. These upwelling zones form important food

chains, with humans at the top. When climatic conditions interfere with their productivity,

the human economy and food supply is severely affected. The most important example

of this concerns the Peru upwelling, caused by a steady offshore wind at the equator,

and is the basis of a huge commercial anchovy ﬁshery.

Upwelling occurs on a smaller scale at the mouths of estuaries when relatively fresh

water entrains deeper salt water. The mixture is still less dense than seawater and ﬂoats

on the surface, carrying nutrient both from the river discharge and from deep entrained

seawater.

Every two to 10 years, the winds responsi ble for the Peru upwelling reverse, in what is

called the southern oscillation. Instead of the upwelling of cold, nutrient-rich water,

warm, nutrient-depleted current appea rs, and the thermocline becomes deeper. The

warm current is cal led the El Nin

o, and it creates havoc with the economy of Peru, as

ﬁshermen lose their anchovy harvest. (El Nin

o is also associated with climatic changes

in regions far from Peru, such as in reducing the number of hurricanes in the Caribbean.)

The phenomenon is referred to as ENSO, for El Nin

o–Southern Oscillation.

Phytoplankton extract carbon, nitrogen, and phosphorus from seawater in the ratio

116 : 16 : 1, respectively. Carbon, of course, is readily available as carbonate. Nitrogen

is more often limiting in marine environments than in freshwater environments, because

nitrogen-ﬁxing cyanobacter (blue-green bacteria) are less abundant in salt water. For this

reason, and becau se phosphorus is recycled faster than nitrogen, phosphorus is less impor-

tant than nitrogen as a limiting nutrient. Silicon can be a limiting nutrient for diatoms.

Figure 15.27 Global distribution of primary productivity in the world’s oceans. (From Barnes and

Mann, 1991. Used with permission.)

556

ECOSYSTEMS AND APPLICATI ONS

Since, as noted previously, silicon cell walls require less energy to form than those made

of cellulose, diatoms are relatively more efﬁcient, and therefore more productive, if sili-

con is sufﬁciently available.

Iron is often the limiting nutrient in the open ocean. As shown in laboratory results in

Table 15.22, iron supplementation of water taken from the Sargasso Sea produced tenfold

increases in productivity when combined with other nutrients. Iron alone produced a

short-term increase in productivity, after which nitrogen and phosphorus limitations

came into play.

A large-scale iron supplementation ﬁeld experiment produced algal blooms in the

Southern Ocean around Antarctica. The experiment was proposed to test a technology

to increase the ocean’s uptake of carbon dioxide, as a way to ameliorate anthropogenic

emissions that could lead to global warming. Since iron is a micronutrient, relatively

small amounts would be necessary to eliminate it as a limiting factor. It could easily be

distributed over a wide area. Although the ﬁeld results support the technical feasibility

of this idea, many other objections remain. These include the possibility of unintended

ecological effects that are likely to result from such a large-scale manipulation of the

environment.

15.4.2 Marine Adaptations

As with wetland animals (see Section 15.3.3), simpler animals tend to be osmoconfor-

mers. This is the case with most invertebrates, such as worms, mussels, and octopi.

Their bodily ﬂuids are isotonic wi th seawater. Therefore, they do not require special

mechanisms to control their internal osmolarity.

Marine ﬁsh are hypotonic: Their body ﬂuids are about one-third as saline as seawater.

Without active mechanisms to counter it, their bodies would lose water by osmosis and

absorb salt. The mechanism is the drinking of large volumes of water, coupled with the

excretion of salt by special ‘‘chloride cells’’ in their gills and by the discharge of a highly

concentrated urine. It is because humans do not have these adaptations that makes it

dangerous for us to drink seawater.

Viscosity becomes an important factor for small organisms. Viscosity is a measure of

the force required to create shear, or velocity gradient, in a ﬂuid. Motion of differing

objects can be compared in terms of the Reynolds number, which is the ratio of momen-

tum to viscosity. If a huma n propels herself through the water with a few strokes o f the

arms, she can expect to coast a short distance because her momentum is great compared to

TABLE 15.22 Nutrient Enrichment Experiment Using

Sargasso Sea Water

Relative Uptake of

C Compared

Nutrient Supplementation to Control (%)

Control (no supplementation) 100

N þ P þmetals, including iron 1290

N þ P 110

N þ P þmetals except iron 108

N þ P þiron 1200

Source: Krebs (1994); original source D. W. Menzel and J. H. Ryther,

1961, Deep Sea Res., Vol. 7, pp. 275–281.

MARINE AND ESTUARINE ECOSYSTEMS

557